The bulldozers are generally provided with the automatic transmission systems. The automatic transmission systems are configured to switch the speed stage up and down in accordance with a vehicle speed and automatically shift down the speed stage especially when the vehicle speed is reduced to a predetermined speed or less due to a load applied to the bulldozers during travelling.
As described in Japanese Patent Nos. 3242421 and 3250820, some of the bulldozers with the aforementioned automatic speed stage shifting function are configured to automatically select the first speed stage when they start moving and subsequently execute a shifting control of shifting up the speed stage to the second speed stage and then to the third speed stage in accordance with increase in a vehicle speed.
By contrast, the so far produced bulldozers include another type of bulldozes configured to start moving at an arbitrary speed stage selected by an operator and subsequently shift up and down the speed stage in accordance with the vehicle speed.
A constant period of speed stage shifting disobedient time is set for the latter type of the bulldozers having the automatic speed stage shifting function. Immediately after starting moving, the bulldozers get accelerated from a vehicle speed of zero. Therefore, a predetermined period of time is set as the speed stage shifting disobedient time that an instruction from the automatic speed stage shifting function to shift down the speed stage is disobeyed in order to prevent the speed stage from being automatically shifted down in this low speed condition during the acceleration period.